This invention relates to a treatment of silica catalyst bases.
Chromium oxide catalysts on a silica-containing support have long been used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. As commercialization of this process became widespread, it became evident that polymer could be produced more economically utilizing a slurry system achieved by carrying out the polymerization at a temperature low enough that the resulting polymer did not go into solution in the diluent used. This process, frequently referred to as a particle form process, while being less complex has certain drawbacks. For one thing, certain control operations which are easily carried out in the solution process are considerably more difficult in the particle form process. Particularly with regard to the molecular weight, it is a simple matter in the solution process to control molecular weight by changing the temperature, with lower molecular weight (higher melt flow) polymer being obtained at the higher temperatures. However, in the slurry process, this technique is inherently limited since any effort to increase the melt flow to any appreciable extent by increasing temperature would cause the polymer to go into solution and thus destroy the slurry or particle form process. This problem has been largely circumvented by catalyst modifications which inherently give a higher melt flow polymer under a given set of conditions. For instance, a small amount of titanium can be coprecipitated with the silica to produce a cogel base and admixed with a chromium compound to provide a catalyst which has the inherent capability of giving higher melt flow polymer. This, however, can create an additional problem in that polymer having a relatively narrow molecular weight distribution is generally produced. Such polymers are ideally suited for some applications but for other applications, a broader distribution is essential. A broader distribution could be obtained by simply forming two different types of catalysts and physically mixing them but this would create two separate polymers which can segregate and thus create a lack of homogeneity in the final product.